The present invention relates to a loudspeaking telephone utilizing a microphone and a loudspeaker, and more particularly, to a loudspeaking telephone with a variable gain circuit for controlling the gains in the receiving and transmitting paths.
In the prior art speakerphones, the gains in the receiving and transmitting paths are switched by means of a variable resistance element, such as a field effect transistor (FET), in response to the values of the receiving and transmitting signal levels. In order to avoid singing or howling, it is preferable for the product of the gains in the receiving and transmitting paths, called as the "total gain", to always be constant.
In the prior art loudspeaking telephones, however, the total gain is not necessarily constant due to the variations of the gate voltage versus conducting resistance characteristics of the FET which determines the gains in the receiving and transmitting paths. Namely, under "a stable period" during which the gains in the receiving and transmitting paths are stable, the total gain is constant. On the contrary, under "a changeover period" during which the gains in the receiving and transmitting paths are switched, the total gain may increase due to the forementioned variations of the characteristics. As a result, the howling or singing problem may occur.
The prior art loudspeaking telephones is described in the following papers:
(1) A. Busala, "Fundamental Consideration in the Design of a Voice-Switched Speakerphone, "THE BELL SYSTEM TECHNICAL JOURNAL, Vo. XXXIX, March 1960, pp. 265-294; and
(2) W. F. Clemency et al, "Functional Design of a Voice-Switched Speakerphone, "THE BELL SYSTEM TECHNICAL JOURNAL, Vo. XL, May 1961, pp. 649-668.
(3) U.S. Pat. No. 4,629,829 issued to Puhl et al, Dec. 16, 1986.